Hydrocyanation catalyst systems, particularly pertaining to the hydrocyanation of olefins, are well known in the art. For example, systems useful for the hydrocyanation of 1,3-butadiene to form 3-pentenenitrile (3PN) and for the subsequent hydrocyanation of 3PN to form adiponitrile (ADN), are known in the commercially important nylon synthesis field. The hydrocyanation of olefins using transition metal complexes with monodentate phosphite ligand is well documented in the prior art. See, for example, U.S. Pat. Nos. 3,496,215; 3,631,191; 3,655,723 and 3,766,237; and Tolman, C. A., McKinney, R. J., Seidel, W. C., Druliner, J. D., and Stevens, W. R., Advances in Catalysis, Vol. 33, page 1 (1985). Improvements in the zero-valent nickel catalyzed hydrocyanation of ethylenically unsaturated compounds with the use of certain multidentate phosphite ligands are also disclosed. Such improvements are described, for example, in U.S. Pat. Nos. 5,821,378; 5,981,772; 6,020,516; and 6,284,865.
The hydrocyanation of activated olefins such as conjugated olefins (e.g., 1,3-butadiene and styrene) and strained olefins (e.g., norbornene) can proceed at useful rates without the use of a Lewis acid promoter. However the hydrocyanation of unactivated olefins, such as 1-octene and 3PN, requires the use of at least one Lewis acid promoter to obtain industrially useful rates and yields for the production of linear nitriles, such as n-octyl cyanide and adiponitrile, respectively.
The use of a promoter in the hydrocyanation reaction is disclosed, for example, in U.S. Pat. No. 3,496,217. This patent discloses an improvement in hydrocyanation using a promoter selected from a large number of metal cation compounds as nickel catalyst promoters with a wide variety of counterions. U.S. Pat. No. 3,496,218 discloses a nickel hydrocyanation catalyst promoted with various boron-containing compounds, including triphenylboron and alkali metal borohydrides. U.S. Pat. No. 4,774,353 discloses a process for the preparation of dinitriles, including ADN, from unsaturated nitriles, including pentenenitriles (PN), in the presence of a zero-valent nickel catalyst and a triorganotin promoter. Moreover, U.S. Pat. No. 4,874,884 discloses a process for producing ADN by the zero-valent nickel catalyzed hydrocyanation of pentenenitriles in the presence of a synergistic combination of promoters selected in accordance with the desired reaction kinetics of the ADN synthesis. Furthermore, the use of Lewis acids to promote the hydrocyanation of pentenenitriles to produce ADN using zero-valent nickel catalysts with multidentate phosphite ligands is also disclosed. See, for example, U.S. Pat. Nos. 5,512,696; 5,723,641; 5,959,135; 6,127,567; and 6,646,148.
A recognized shortcoming of the catalyst systems and processes described above is the inability to hydrocyanate the conjugated 2-pentenenitrile isomers, 2PN. U.S. Pat. No. 3,564,040 describes that 3PN is slowly isomerized to 2PN during the hydrocyanation process, and the 2PN so produced is treated as a yield loss. Furthermore, 2PN has been shown to be both a catalyst inhibitor and a catalyst poison as the concentration increases. In order to mitigate this poisoning effect, 2PN is typically separated before recovered pentenenitrile is recycled to the reactor.
In order to address the negative effects of 2PN, U.S. Pat. No. 3,564,040 describes a method to maintain the steady-state concentration of 2PN below 5 mole percent as based on the nitriles present in the reaction mixture. Because trans-2PN is difficult to separate from a mixture of 3PN and 4PN by distillation due to their close relative volatilities, the disclosed method involves the catalytic isomerization of trans-2PN to cis-2PN followed by fractional distillation of the mixture of PN isomers to remove the more volatile cis-2PN isomer. The catalyst systems used to isomerize trans-2PN to cis-2PN are those that also serve to hydrocyanate PN to ADN, in particular, nickel catalysts derived from monodentate phosphite ligands as described in U.S. Pat. Nos. 3,496,217 and 3,496,218.
Alternative catalyst systems for the isomerization of trans-2PN to cis-2PN are disclosed in U.S. Pat. Nos. 3,852,325 and 3,852,327. The primary advantage of the catalyst systems described therein is in avoiding appreciable carbon-carbon double bond migration in the PN isomers, which allows for the isomerization of trans-2PN to cis-2PN without substantial further isomerization of the 3PN to 2PN. The catalysts described in U.S. Pat. No. 3,852,325 are compounds of the general formula R3C—X, such as triphenylmethyl bromide, wherein R is an aryl radical having up to 18 carbon atoms and —X is of the group consisting of —H, —Cl, —Br, —I, —SH, —B(C6H5)4, —PF6, AsF6, —SbF6 and —BF4, while the catalyst systems described in U.S. Pat. No. 3,852,327 are Lewis acid/Lewis base compositions, such as combinations of zinc chloride with triphenylphosphine.
A different method of removing the 2PN from mixtures of PN isomers containing 3PN and 4-pentenenitrile (4PN) is disclosed in U.S. Pat. No. 3,865,865. The 2PN and/or 2-methyl-2-butenenitriles (2M2BN) can be selectively separated from a mixture of PN isomers containing 3PN and 4PN by contacting the mixture of nitriles with an aqueous solution of a treating agent comprising sulfite and bisulfite ions and ammonium or alkali metal cations to produce an aqueous phase containing the bisulfite adduct of the 2PN and/or 2M2BN and an organic phase containing the 3PN and 4PN, substantially free of 2PN or 2M2BN. The recovered organic phase can provide a feed material of PN for further hydrocyanation to produce adiponitrile with greatly reduced amounts of the undesired by-product 2PN that are detrimental to catalyst efficiency.
Recently, a class of hydrocyanation catalysts comprised of zero-valent nickel and a bidentate phosphite ligand have been described that are generally more active than the hydrocyanation catalyst comprised of monodentate phosphites and nickel. As a result, this class of catalysts may be used effectively at much lower concentrations and over a broader range of reaction conditions. U.S. Pat. No. 5,688,986 reveals that at least one member of this class of catalysts are capable of hydrocyanating olefins conjugated to nitriles, for example 2PN. However, we have observed that this ability is not a general feature of this class of catalysts. It therefore would be desirable to identify hydrocyanation catalyst systems that can be resistant to the inhibiting and poisoning effects of 2PN. Also desirable would be processes which use such catalyst systems to produce the valuable products 3PN, 4PN, and/or ADN from 2PN, such as by the isomerization of 2PN to form 3PN and/or 4PN and by the hydrocyanation of 2PN to form ADN.